An X-ray image pickup device that uses a flat panel detector (Flat Panel Imager: referred to as “FPI,” hereinafter) has been put to practical use. The X-ray image pickup device that uses the FPI has some excellent advantages, such as having better sensitivity and image quality than an X-ray image pickup device that uses a film and being able to take a moving picture.
As shown in FIG. 8, the FPI has a pixel 80 that includes a photoelectric conversion element 11 and a TFT (Thin Film Transistor) 12, which serves as a switching element. The TFT 12 is connected to a gate line 13, which is used to switch the TFT, as well as to a signal line 14, which is used to read a signal. The photoelectric conversion element 11 is connected to a bias line 15.
A TFT array substrate is formed by disposing a plurality of the above pixels 80 two-dimensionally on a glass substrate.
After a human body is irradiated with X-rays from an X-ray source, a fluorescent layer in the FPI converts the X-rays that have passed through the human body into visible light. Electrical charge is accumulated in the photoelectric conversion element 11 of the TFT array substrate. As the TFT 12 is turned ON (ON; closed circuit state), the accumulated charge is read from the photoelectric conversion element 11, and then is output via the signal line 14 as charge signals.
The charge signals are converted to electric signals. The electric signals are amplified by a plurality of signal amplification circuits before being converted by an analog/digital converter (A/D converter) into digital signals. The image data that have been converted into digital signals are processed by an image processing circuit into a moving image, which is then displayed on a monitor. An X-ray moving image pickup device is controlled by a control personal computer that is equipped with an image processing device, a program/control board, and the like. Video signals are operated repeatedly with the above process as one frame (See Patent Document 1, for example).
FIG. 10 shows a timing chart of the driving of the TFT. The drive timing of the TFT has the following three timings: a period a during which electric charge is accumulated within one frame of image signals in response to X-rays; a period b during which the charge is read; and a period c during which the charge is refreshed.
In this manner, the charge that is accumulated in the period during which the charge should be accumulated is read from the photoelectric conversion element 11 after the TFT 12 is turned ON. However, the TFT 12 is not an ideal switch element. Therefore, some electric charge remains. Since the TFT 12 has a finite resistance value RON (several mega ohms), the time constant that is determined between the TFT 12 and the capacitance CPD of the photoelectric conversion element 11 puts a restriction on the amount of charge that can be read.
Accordingly, in order to read a sufficient amount of charge, the time during which the TFT 12 is ON, or the charge reading period, needs to be longer. During the period, as the time during which the TFT 12 is ON becomes longer, the time during which the X-rays can be accumulated as signals (charge accumulation period) becomes shortened. As a result, sufficient output power cannot be obtained. On the other hand, if the time during which the TFT 12 is ON is short, a sufficient amount of charge cannot be read. As a result, the charge still remains in the next frame.
Therefore, an operation of switching the TFT 12 again to release the charge after the charge is read by turning the TFT 12 ON is carried out. The operation is called refresh.
Even during the refresh, the time during which the TFT 12 is ON needs to be long enough to release a sufficient amount of electric charge.
FIG. 9 shows an example of a conventional refresh operation method.
According to the conventional refresh operation method, if the number of gate lines, G1 to Gn, is n, one pulse of ON-control signal is input into G1 with an interval of 2TON.
Then, the timing is so controlled that the control signal of G2 is turned ON at a time when the control signal of G1 is turned OFF (OFF; closed circuit state). In this manner, one pulse of ON-control signal is similarly input with an interval of 2TON.
Similarly, one pulse of control signal is sequentially input into G3, G4, . . . and then Gn. In this manner, the refresh operation is performed.
However, according to the conventional refresh operation method, if the time during which the TFT is ON is doubled or quadrupled, the total time required for the refresh becomes doubled or quadrupled as well.
That is, in the case of the conventional refresh timing, if the ON time for refresh is 2TON, and the number of TFTs connected to the gate lines is n, the time required for the refresh is 2TON×n. Therefore, as the ON time becomes longer, the time becomes longer in proportion to the number of TFTs connected to the gate lines.
It is preferred that electric charge be released sufficiently even during a short refresh period. As a means for achieving the sufficient release of charge, a method of changing the polarity of voltage applied to the photoelectric conversion element 11, or the like is added.
However, according to the above method, the polarity of voltage applied to the photoelectric conversion element 11 needs to be changed on a per-frame basis. Therefore, the lack of stability in the operation is a problem.
Meanwhile, if the time during which the gate of the TFT 12 is ON is simply extended to ensure the time required for reading electric charge, and if an adjoining gate is opened even when a given gate is still opened, there is an increase in power consumption. Moreover, the electric charge at a time when the TFT 12 is ON is induced to a pixel that has been already turned ON. Accordingly, a large offset takes place. Therefore, the video turns unnatural.